From patent No. EP 1995745 a screw is known designed for the same purpose, the said screw having the main part made of material other than copper or copper alloys, mostly of steel, and provided with a thread and a head with a bore which is positioned coaxially within the screw and has side walls convergent towards the bore inlet, and a bottom in the form of a convex truncated cone. In the head bore an insert made of copper or copper alloy is seated in such a way that it remains in close contact with the bore walls and it is also seated in a circumferential groove located at a certain distance from the bore inlet in the top surface of the head. Preferably, the top surface of the insert is provided with grooves.
A method of manufacturing the screw consists in that the main part and the head of the screw are formed from a bar stock, a rolled wire rod or an extruded wire rod made by cold forging, and further the bore is produced which extends along the screw axis and has the bottom in the form of a convex truncated cone. Then the insert made of copper or copper alloy is introduced into the bore and, upon the action of appropriate pressure, the insert is seated in the bore in such a manner that the insert material fully fills the entire bore, and grooves are formed in the top surface of the insert. The element shaped in such a way is subjected to plastic working using a suitable tool which forms a bore with side walls convergent towards the bore inlet and a circumferential groove in the top surface of the head at a certain distance from the bore inlet.
The screw obtained in the process described above ensures permanent seating of the copper or copper alloy insert in the head of the screw whose remaining part is basically made of steel; however, the electrochemical corrosion caused by the difference in electrochemical potentials between copper or copper alloys and steel cannot be avoided and, therefore, electrical properties of electric contact between the insert and the head progressively worsen during the use of the screw mounted in the starter.